In recent years, letter printers have come into wide use with the development of office automation.
In letter printers, a memory of a computer-controlled image including letters is output, namely, printed on a paper by an image-forming mechanism of electrophotography or electrographic recording. That is, an electrostatic latent image formed on a photosensitive plate of photoconductor by electrophotography or on a dielectric master by electrographic recording, is developed by a toner, whereby the latent image is transferred and fixed on a paper as a visualized toner image and an image print is obtained. These letter printers, being able to print even complicated letters like most Chinese characters, have a pet name of "kanji printers" (kanji refers in Japanese language to Chinese characters) in Japan.
Toner image formation, namely, development is conducted by a dry method or by a wet method (the wet method is also called a liquid drying method). A toner image transferred onto a paper is melt-fixed (thermally fixed) in the dry method by being heated to several hundreds centigrade (e.g. 200.degree. to 300.degree. C.). In the wet method, there are transferred onto a paper not only a toner image but also a solvent of a developing solution and, therefore, the toner image is fixed with heating (this heating also serves to vaporize the solvent).
As the paper for letter printers, there have hitherto been used plain papers as well as slightly processed papers. Recently, carbonless papers have come to be used. This is because printing of formats of account books, chits, etc. can be made by a letter printer, and printing of letters can be made by an impact printer or hand writing and copying.
In carbonless papers, various chemicals are coated as mentioned below. This causes many troubles during printing on these papers by a letter printer. Such troubles are summarized as follows.
1. Since the photosensitive plate or master of a letter printer contacts directly with the surface of a lower (or intermediate) sheet of carbonless paper where an acidic substance is coated, there are cases that the plate or master is chemically attacked after repeated use.
2. Since carbonless papers are exposed to high temperatures at the time of thermal fixation of a transferred toner image particularly in the dry method, there are cases that the lower (or intermediate) sheet of carbonless paper produces the smell and/or fume of chemicals.
3. Particularly in wet (liquid drying) development, there are cases that microcapsules coated on the upper (or intermediate) sheet of carbonless paper are destroyed by a solvent of a developing solution.